An ultrasonic transducer uses ultrasound to perform sensing. A piezoelectric vibration element in the ultrasonic transducer intermittently transmits ultrasonic pulse signals and receives waves reflected from surrounding obstacles, and thereby detects an object.
An ultrasonic transducer of this type is disclosed in Patent Document 1. FIG. 1 is a cross-sectional view illustrating a housing structure of an ultrasonic transducer 1 disclosed in Patent Document 1. A housing 11 of the ultrasonic transducer is a bottomed cylindrical housing that is circular in transverse section. A piezoelectric element 12 is mounted on a center of an inner surface of a bottom of the housing 11. The bottom of the housing 11 is thick at the center on which the piezoelectric element 12 is mounted, and gradually becomes thinner toward an inner wall surface of the housing 11.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2006-174003
The ultrasonic transducer disclosed in Patent Document 1 has characteristics of less reverberation and narrow directivity.
Referring to FIG. 1, a portion indicated by A (i.e., a corner between a slope portion of the bottom of the housing and a side wall) has a sharp edge in design. However, it is difficult in practice to form a sharp edge at this location.
The portion indicated by A in FIG. 1 is rounded with a curvature radius R in practice. It has been found that variations in curvature radius R cause significant variations in resonant frequency of the ultrasonic transducer, and contribute to unstable characteristics of the ultrasonic transducer.
It has also been found that the levels of amplitude and sensitivity provided by the structure of FIG. 1 are not satisfactory.